1. Field of the Invention
The invention relates to a safety circuit for surveillance of a wheel lockup-protected motor vehicle brake system.
2. Brief Description of the Background of the Invention Including Prior Art
The output values of pick-up transducers, disposed at motor vehicle wheels to be controlled, are fed to an automatic control electronic in a wheel lockup-protected motor vehicle brake systems. The pick-up transducers, in general furnish voltage signals, where the frequency of the voltage signals is proportionate to the speed of the motor vehicle. In this case, the pick-up transducers form in general a magnetic field, which is influenced by a toothed magnet wheel passing by at a small distance. The toothed magnet wheel induces a voltage in the coil of the pick-up transducer.
The pick-up transducers are disposed at the wheels of the motor vehicle and thus are subjected to substantial loads based on vibrations, jolts, and shocks of the motor vehicle and are further subject to soiling, which can substantially influence the signal generation.
It is thereby possible that voltage signals are generated, which do not correspond to the actual speed of the motor vehicle wheel. Since such defective capturing or recording of the speed of the motor vehicle wheel can lead to dangerous situations during application of a braking control, safety and surveillance or monitoring circuits are generally provided which, upon occurrence of such errors in the wheel lockup-protected brake system, recognize and signal these errors and, if necessary, switch off the defective subpart regions of the wheel lockup-protected brake system, such that the brake system then operates only like a conventional brake system.
A safety circuit is known from the printed flyer "WABCO-Anti-Blockier-System, Erhohte Sicherheit durch Diagonal-Aufteilung," (WABCO Antilock System, Increased Safety by Diagonal Partitioning), Edition Aug. 1983, Publisher WABCO Westinghouse Fahrzeugbremsen GmbH, Hanover. This safety circuit controls the pick-up transducers and the respective cabling or cable circuits upon the start of driving and during the driving. Said safety circuit signals the occurring errors to the driver and switches off simultaneously the defective part of the system in a diagonal fashion. In this case, only the part of the automatic control electronic for two diagonally, oppositely disposed motor vehicle wheels of different motor vehicle axles is switched off.
It is conceivable in the context of an antilock system with such a safety circuit that, upon starting up of a large commercial vehicle or upon shifting of gears or of shifting into gear, upon loading or other vibrations, there will occur a short-term wheel motion of individual motor vehicle wheels, which motion is evaluated by the control electronics as a jump in velocity. Since such jumps in velocity can be interpreted as sudden, disproportionately large speed changes, which cannot occur during normal driving conditions or usual motor vehicle operation, the safety circuit evaluates this as error of the wheel lockup-protected brake system and therefore switches off the wheel lockup-protected brake system. Thus, only the conventional brake system remains in operation which however, offers less driving safety versus the wheel lockup-protected brake system. Since, however, such vibration-caused interferences do not represent a proper error of the brake system, it is disadvantageous if the wheel lockup-protected brake system is switched off in such cases.